Basalt: Igneous Rock - Pictures, Definition, Uses & More Basalt What Is Basalt, How Does It Form, and How Is It Used? Basalt: A fine-grained igneous rock that is usually black in color. The specimen shown is about two inches (five centimeters) across. What is Basalt? Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro . The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is a coarse-grained rock. Olympus Mons Volcano: This shield volcano is composed of basalt and has enormous calderas at the summit. Olympus Mons is the highest topographic feature on Mars and is the largest known volcano in our solar system. It is about 375 miles (600 kilometers) in diameter and 15 miles (25 kilometers) high. NASA Mars Orbiter Camera image.

Earth's Most Abundant Bedrock Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock. Basalt on Moon and Mars Basalt is also an abundant rock on the Moon. Much of the Moon's surface is underlain by basaltic lava flows and flood basalts. These areas of the Moon are known as "lunar maria." Large areas of the Moon have been resurfaced by extensive basalt flows which may have been triggered by major impact events. The ages of lunar maria can be estimated by observing the density of impact craters on their surface. Younger basalt flows will have fewer craters. Olympus Mons is a shield volcano on Mars. It, like most other volcanic features on Mars, was formed from basaltic lava flows. It is the highest mountain on Mars and is the largest known volcano in our solar system. Basalt-Forming Environments: This map shows the location of oceanic divergent boundaries and hotspots.

These are locations where large volumes of basalt have been formed. Map copyright by Geology.com and MapResources. Locations generalized after United States Geological Survey, Geologic Investigations Map I-2800: This Dynamic Planet. Igneous rock composition chart: This chart shows that basalt is typically composed of pyroxenes, plagioclase, micas, and amphiboles. Basalt-Forming Environments Most of the basalt found on Earth was produced in just three rock-forming environments: 1) oceanic divergent boundaries , 2) oceanic hotspots , and 3) mantle plumes and hotspots beneath continents . The images on this page feature some of these basalt-forming environments. Sea floor pillow basalts on the Juan de Fuca Ridge, a divergent plate boundary located about 150 miles (240 kilometers) west of the Washington-Oregon coast. This lava flow, produced by a fissure eruption, was about five years old when the photograph was taken. NOAA Ocean Explorer image. Hawaii Basalt Flows: Lava flows dump into the Pacific Ocean on the coast of Hawaii.

Multiple locations where hot lava streams into the ocean can be seen in this image along with a red-hot lava flow traversing the lava field. This photo shows the enormous extent of the flows. They extend from the shoreline up to the horizon. A volcanic plume from the Pu`u `O`o vent can be seen over the horizon near the center of the image. The lava in these flows originated from the Pu`u `O`o vent. USGS image. Basalts at Oceanic Divergent Boundaries Most of Earth's basalt is produced at divergent plate boundaries on the mid-ocean ridge system (see map). Here convection currents deliver hot rock from deep in the mantle. This hot rock melts as the divergent boundary pulls apart, and the molten rock erupts onto the sea floor. These submarine fissure eruptions often produce pillow basalts as shown in the image on this page. The active mid-ocean ridges host repeated fissure eruptions. Most of this activity is unnoticed because these boundaries are under great depths of water. At these deep locations, any steam, ash, or gas produced is absorbed by the water column and does not reach the surface.

Earthquake activity is the only signal to humans that many of these deep ocean ridge eruptions provide. However, Iceland is a location where a mid-ocean ridge has been lifted above sea level. There, people can directly observe this volcanic activity. Thermal image of a hot basalt flow on the flank of Hawaii's Kilauea volcano. Hot lava at the front of the flow is revealed in yellow, orange and red colors. The channel that it flowed through on the previous day appears as a purple and blue track. United States Geological Survey image. Oceanic Hotspots Another location where significant amounts of basalt are produced is above oceanic hotspots. These are locations (see map above) where a small plume of hot rock rises up through the mantle from a hotspot on Earth's core. The Hawaiian Islands are an example of where basaltic volcanoes have been built above an oceanic hotspot. Basalt production at these locations begins with an eruption on the ocean floor. If the hotspot is sustained, repeated eruptions can build the volcanic cone larger and larger until it becomes high enough to become an island. All of the islands in the Hawaiian Island chain were built up from basalt eruptions on the sea floor.

The island that we know today as "Hawaii" is thought to be between 300,000 and 600,000 years old. It began as an eruption on the floor of the Pacific Ocean. The volcanic cone grew as recurrent eruptions built up layer after layer of basalt flows. About 100,000 years ago it is thought to have grown tall enough to emerge from the ocean as an island. Today it consists of five overlapping volcanoes. Kilauea is the most active of these volcanoes. It has been in amost continuous eruption since January, 1983. Basalt flows from Kilauea have extruded over one cubic mile of lava, which currently covers about 48 square miles of land. These flows have travelled over seven miles to reach the ocean, covering highways, homes and entire subdivisions that were in their path. Columbia River Flood Basalts: The Columbia River Flood Basalts are an extensive sequence of stacked lava flows that reach a cumulative thickness of up to 6000 feet. The outcrops in the foreground and in the distance of this photo are all made up of layered basalt flows.

Although basalt is typically a dark black rock, it often weathers to a yellow-brown color similar to the rocks shown here. Public domain image by Williamborg. Columbia River Flood Basalts Map: A map of the area underlain by the Columbia River Flood Basalts in Washington, Oregon, and Idaho. The area shown is what has not yet been eroded away - the original extent of these basalt flows was much greater. Over 300 individual flows have been identified, and several hundred meters of basalt underlies much of the area shown in the map above. Map © by Geology.com and MapResources.com. Plumes & Hotspots Below Continents The third basalt-forming environment is a continental environment where a mantle plume or hotspot delivers enormous amounts of basaltic lava through the continental crust and up to Earth's surface. These eruptions can be from either vents or fissures. They have produced the largest basalt flows on the continents. The eruptions can occur repeatedly over millions of years, producing layer after layer of basalt stacked in a vertical sequence (see outcrop photo).

The Columbia River Flood Basalts in Washington, Oregon, and Idaho are an example of extensive flood basalts on land (see map below). Other examples include the Emeishan Traps of China, the Deccan Traps of India, the Keweenawan Lavas of the Lake Superior region, the Etendeka Basalts of Namibia, the Karroo Basalts of South Africa, and the Siberian Traps of Russia. (The word "traps" is derived from the Swedish word for "stairs," which describes the outcrop profile of these layered basalt deposits, as shown in the outcrop photo.) Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials. The best way to learn about rocks is to have specimens available for testing and examination. The Roman theatre: (left) in Bosra, Syria. The dark building stone is basalt. Image © iStockphoto / Steve Estvanik. Basalt paving stones: (right) on a city street in Rome, Italy. Basalt pavers were often used in areas close to volcanoes. Image © iStockphoto / Giovanni Rinaldi.

Uses of Basalt Basalt is used for a wide variety of purposes. It is most commonly crushed for use as an aggregate in construction projects. Crushed basalt is used for road base, concrete aggregate, asphalt pavement aggregate, railroad ballast, filter stone in drain fields, and may other purposes. Basalt is also cut into dimension stone. Thin slabs of basalt are cut and sometimes polished for use as floor tiles, building veneer, monuments, and other stone objects. Basalt and Granite Ask GeoMan... What are the most important types of rock in the crust? Excluding the rocks between my ears, I'd have to say that basalt and granite have the honor of being the most important rocks in the crust. Basalt and granite actually have quite a bit in common. Both are igneous rocks , which means that they cooled from a magma (the earth gets very hot just below the surface, and there is lots of liquid rock available). Both are made up of minerals from the silicate group, so both have large amounts of silicon and oxygen . Both will hurt if you drop a big piece on your toe. But there are several important differences, too. These differences help define and explain how the earth works. Granite is great stuff!

Not only is it my personal favorite, it is without a doubt the most common rock type on the continental land masses. Yosemite Valley in the Sierra Nevada and Mt. Rushmore are two notable examples of granitic rocks. But granitic "basement rock" can be found just about everywhere east of the Rockies if you're willing to dig through the dirt and sedimentary rocks at the surface. Granite is intrusive, which means that the magma was trapped deep in the crust , and probably took a very long time to cool down enough to crystallize into solid rock. This allows the minerals which form plenty of time to grow, and results in a coarse-textured rock in which individual mineral grains are easily visible. Granite is the ultimate silicate rock. As discussed elsewhere in greater detail , on average oxygen and silicon account for 75% of the earth's crust. The remaining 25% is split among several other elements, with aluminum and potassium contributing the most to the formation of the continental granitic rocks. Relatively small amounts of iron and magnesium occur, but since they have generally higher densities it's not surprising that there isn't very much in the granite.

Due to the process of differentiation , most of the heavier elements are moving towards the core of the earth, allowing the silicon and oxygen to accumulate on the surface. And accumulate it has. Enough granitic "scum" has differentiated to the surface to cover 25% to 30% of the earth with the good stuff. We call this purified material felsic because of the relatively high percentage of silica and oxygen. Basalt is extrusive. The magma from which it cools breaks through the crust of the earth and erupts on the surface. We call these types of events volcanic eruptions , and there are several main types. The volcanoes that make basalt are very common, and tend to form long and persistent zones of rifting in nearly all of the ocean basins. We now believe that these undersea volcanic areas represent huge spreading ridges where the earth's crust is separating. It's a lot like a cut on your arm, which will bleed until a scab forms. Basaltic magma is like the blood of the earth - it's what comes out when the earth's skin is cut the whole way through.

As an eruption ends, the basalt "scab" heals the wound in the crust, and the earth adds some new seafloor crust. Because the magma comes out of the earth (and often into water) it cools very quickly, and the minerals have very little opportunity to grow. Basalt is commonly very fine grained , and it is nearly impossible to see individual minerals without magnification. Basalt is considered a mafic silicate rock. Among other characteristics, mafic minerals and rocks are generally dark in color and high in specific gravity . This is in large part due to the amount of iron, magnesium, and several other relatively heavy elements which "contaminate" the silica and oxygen. But this heavy stuff really isn't happy near the surface, and will take any opportunity it can to head for deeper levels . The trick is to heat the basalt back up again so it can melt and give the iron another shot at the core. It wants to be there, and heat is the key which unlocks the door. As it turns out, most of the ocean floor is basalt, and most of the continents are granite.

Basaltic crust is dark and thin and heavy, while granite is light and accumulates into continent-sized rafts which bob about like corks in this "sea of basalt." When a continent runs into a piece of seafloor , it's much like a Mac truck running into a Volkswagon. Not very pretty, but at least there's a clear winner. And the seafloor basalt ends up in pretty much the same position as does the VW - under the truck (or continent, as the case may be). This may seem like a drag for the basalt, but remember that it isn't all that happy on the surface anyway, and this gives it the heat it needs to re-melt and try to complete the differentiation process which was so rudely interrupted at the spreading ridge. If successful and allowed to continue, what's left behind is a "purified" magma with most of the iron, magnesium, and other heavy elements removed. When it cools, guess what forms? And the continental land mass just got a wee bit larger.

Igneous Rocks | Pictures of Intrusive and Extrusive Rock Types Pictures of Igneous Rocks Photos of Common Intrusive and Extrusive Igneous Rock Types Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende , pyroxene, and biotite . The specimen shown is about two inches (five centimeters) across. What are Igneous Rocks? Igneous rocks are formed from the solidification of molten rock material. There are two basic types. Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite , gabbro , granite , pegmatite , and peridotite . Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite , basalt , obsidian , pumice , rhyolite , scoria , and tuff . Pictures and brief descriptions of some common igneous rock types are shown on this page.

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende , and sometimes quartz . The specimen shown above is about two inches (five centimeters) across. Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across. Gabbro is a coarse-grained, dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine . The specimen shown above is about two inches (five centimeters) across. Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across. Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz , feldspar, and mica minerals. The specimen above is about two inches (five centimeters) across. Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine .

It may contain small amounts of amphibole, feldspar, quartz , or pyroxene. The specimen shown above is about two inches (five centimeters) across. Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber. The specimen shown above is about two inches (five centimeters) across. Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. The specimen shown above is about two inches (five centimeters) across. Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. The specimen shown above is about two inches (five centimeters) across. Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification.

It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne. The specimen shown above is about two inches (five centimeters) across. Fire Opal is sometimes found filling cavities in rhyolite . Long after the rhyolite has cooled, silica-rich ground water moves through the rock, sometimes depositing gems like opal , red beryl , topaz , jasper, or agate in the cavities of the rock. This is one of many excellent geological photographs generously shared through a Creative Commons License by Didier Descouens. The best way to learn about rocks is to have a collection of specimens to examine while you study. Seeing and handling the rocks will help you understand their composition and texture much better than reading about them on a website or in a book. The Geology.com store offers inexpensive rock collections that can be mailed anywhere in the United States or U.S. Territories. Mineral collections and instructive books are also available. Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock.

It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders. The specimen shown above is about two inches (five centimeters) across. More Rocks Interactives . The Rock Cycle . Types of Rocks Types of Rocks Rocks are not all the same! The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed. Sedimentary Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone. Metamorphic Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing).

The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble. Igneous Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian. Here's a chart of some of the key characteristics that can help you identify the rocks within these three main classes. Crystals Small, flat surfaces that are shiny or sparkly, like tiny mirrors. Fossils Imprints of leaves, shells, insects, or other items in the rock. Gas bubbles "Holes," like Swiss cheese, in the rock. Glassy surface A shiny and smooth surface, like colored glass. Ribbonlike layers Straight or wavy stripes of different colors in the rock.

Sand or pebbles Basalt - Igneous rocks About Basalt Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene ( augite ) and iron ore (titaniferous magnetite ). Basalt may also contain olivine , quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro . Basalt is a fine-grained and dark-colored rock. Black color is given to basalt by pyroxene group mineral augite. Width of the sample is 12 cm. Basalt is usually black or dark gray and relatively featureless. It is composed of mineral grains which are mostly indistinguishable to the naked eye. Basalt may also contain volcanic glass. Basalt may contain phenocrysts (larger crystals within fine-grained groundmass) and vesicules (holes that were filled by volcanic gases ). Black color is given to basalt by pyroxene and magnetite. Both of them contain iron and this is the reason why they are black. So this is iron again which is responsible for the coloration of basalt.

Plagioclase, volumetrically usually the most important constituent, is mostly pale gray in color. Basalt lava flowing in Hawai’i (Kilauea volcano, Pu’u O’o vent). Basalt is a major rock type that occurs in virtually every tectonic setting. Basalt is clearly the most common volcanic rock on Earth and basaltic rocks (including gabbro, diabase and their metamorphosed equivalents) are the most common rocks in the crust 2 . Basalt is also common on the Moon and other rocky planets of the Solar System. What makes basalt so common? Basalt is the original constituent of the crust from which almost all other rock types have evolved. Basalt forms when mantle rocks ( peridotite ) start to melt. Rocks melt incongruently. It basically means that melt that forms has a different composition from the source rocks. Of course, it can only happen if rocks melt only partially, but this is exactly what happens in the upper mantle.

It melts partially to yield basaltic magma which is less dense and rises upward to form new oceanic crust in mid-ocean ridges or volcanoes and intrusives (dikes, sills) in many other tectonic regimes. Basalt is the source rock of other more evolved volcanic rocks like dacite, rhyolite, etc. Basalt pebbles near the southern tip of La Palma slowly transforming into black sand typical to volcanic oceanic islands. Basalt sample collected near the Giant’s Causeway, Northern Ireland. Width of sample 8 cm. Gabbro is a coarse-grained (intrusive) equivalent of basalt. This sample of gabbro comes from La Plama. La Palma is an oceanic island, but some parts of it are uplifted and there are deep ravines like Caldera de Taburiente that cuts deep into the interior of the island and allows intrusive rocks like gabbro to be exposed. Width of sample 10 cm. Basaltic rocks may carry xenoliths from the mantle. Here is a bright green dunite xenolith inside basalt from Hawai’i. Width of sample 8 cm. Classification Basalt has a strict chemical definition.

It is defined in the TAS diagram shown above. Basalt is an igneous rock that contains more than 45 and less than 52% of SiO2 and less than five percent of total alkalies (K2O + Na2O) 3 . Neighboring rock types like basaltic andesite, basanite, picrite (picrobasalt), trachybasalt and even more distant rocks like phonotephrite or andesite may have very similar look and can be easily mistaken for basalt in many cases. Basalt is widespread in many tectonic regimes, but there are slight variations in chemical composition which allow more precise classification. MORB is an acronym for “mid-ocean ridge basalt” and OIB for “oceanic island basalt”. MORB is a result of partial melting of the upper mantle which is already recycled many times while OIB is at least partly from more deeper part of the mantle (deep-sourced mantle plumes that feed hot spots like Hawai’i or the Canary Islands) and is therefore less depleted in incompatible chemical elements. Andesite is similar to basalt, but it contains more silica and is generally lighter in color.

White crystals are plagioclase phenocrysts, but they contain less Ca and more Na than plagioclase in basalt does. Andesite is very common product of subduction zone volcanism. Santorini, Greece. Width of sample 7 cm. Composition Average chemical composition of basalt determined by 3594 chemical analyses of basaltic rocks 2 (numbers are mass percents, recalculated volatile-free to total 100%): SiO2 — 49.97 K2O — 1.12 P2O5 — 0.35 Minerals that host these chemical elements (chemical composition of igneous rocks is traditionally expressed in oxides) are augite, plagioclase and titaniferous magnetite. These minerals are difficult to demonstrate because they are too small to be seen in typical basalt, but some basaltic rocks are porphyritic (lots of porphyritic rocks can be seen here: porphyry ) and show some of these minerals nicely (unfortunately not magnetite, though). Basalt porphyrite from the Isle of Mull, Scotland with many plagioclase phenocrysts. The rock is 8 cm in length.

Porphyritic basaltic rock from Tenerife. Phenocrysts are plagioclase (white) and augite (black). Width of sample 14 cm. Magnetite crystals are always microscopic in basalt, but sometimes they form black stripes in light-colored sand . Here are heavy minerals (mostly magnetite) as a residue of weathering of basaltic rocks. White Park Bay, Northern Ireland. Basaltic rock (most likely basanite) from Caldera de Taburiente, La Palma. Black is pyroxene augite, orange is olivine or more precisely what is left of it. Orange patches are former olivine crystals that are now composed of a mixture of silicates and iron oxides which is known as iddingsite. Olivine is a common mineral in many basaltic rocks. Width of view 10 cm. Another basalt (chemically probably picrobasalt) with lots of olivine (fresh olivine is bright green, but it gets more and more yellow as it weathers). Oahu, Hawai’i. Width of sample 6 cm.

Basalt in the field Subaerial basalt forms lava flows or pyroclastic fields and cones. Two main types of basaltic lava flows are aa lava and pahoehoe lava . Aa lava has rough rubbly irregular crust while pahoehoe is smooth. Lava crust of aa type is broken into pieces while pahoehoe retains its continuity. Both lava flow types are massive beneath the crust and this massive interior may be columnar. Columns are separated from each other by narrow cracks which form because cooling basaltic magma contracts. Cracks start to form at the surface and propagate deeper as lava cools. Submarine basalt usually forms pillows. Pillow basalt forms as a result of very rapid cooling. Outer part of forming pillow cools very quickly in contact with cold seawater while the interior still fills with molten lava. Basalt mostly forms lava flows because it is among the least viscous magma types and therefore does not generate explosive volcanic eruptions, but sometimes pyroclastic material is formed when magma contains more volcanic gases. Basaltic rocks can be thrown out of volcanic vents as lapilli (singular: lapillus) and volcanic bombs.

Basaltic volcanoes are fed by dikes (planar intrusive rock bodies when solidified that cut through other rocks) and sills (similar to dike but generally parallel to preexisting bedding planes). Basaltic lava flows of Kilauea volcano in Hawai’i. Aa lava in the foreground. La Palma, Canary Islands. Pahoehoe lava (ropy lava). La Palma, Canary Islands. Basalt columns. Giant’s Causeway, Northern Ireland. Pillow lava near Fasoula, Troodos ophiolite, Cyprus. Pillow lava is very common on Earth, but difficult to find because almost all of it is on the ocean floor. Examples can be found on land usually where former ocean floor is tectonically squeezed between two blocks of continental crust. Scoriaceous lapillus from Etna, Italy. Despite being 5 cm in width it weighs only 15 grams because it is filled with gas bubbles (vesicules). Similar rock type with a felsic composition is pumice . Sometimes dikes are so close to each other that the whole outcrop is composed of them. These sheeted dikes in Cyprus once fed volcanoes on the ocean floor.

Dikes are composed of basalt and diabase. Diabase is nothing more than coarse-grained basalt. Here is a contact between basalt (on the left) and diabase in Cyprus. The basaltic dike is fine-grained because it is younger and was chilled (it lost heat rapidly to the diabase dike on the right). Columns in basalt are perpendicular to the cooling front. In this case it is evident that basalt formed a tube (filled lava tunnel). Such conduits are common phenomena in volcanic islands and provide a way for the volcano to enlarge itself because magma can flow great distances inside such thermally insulated tubes before solidifying. Tenerife, Canary Islands. Dikes and sills are often visible on the ground and may become notable landforms. Salisbury Crags in Edinburgh is basaltic sill. Metamorphism and weathering Basalt is largely composed of minerals with little resistance to weathering. Hence, basalt as a whole also tends to disintegrate faster than granite and other felsic rock types. Magnetite is one of the most resistant common minerals in basalt and forms the bulk of heavy mineral sands .

Other minerals disintegrate and release their components to water as ions or form clay minerals. Iron and aluminum are among the least mobile ions and therefore tend to form laterite deposits enriched in these elements. Basalt metamorphoses to a number of different rock types, depending on pressure, temperature, and the nature of volatile compounds that react with minerals in basalt. Most common metamorphic rocks with basaltic protolith are chlorite schist, amphibolite , blueschist, and eclogite . Black sand forms in volcanic islands when quartz and biogenic grains are not available. Here is a basaltic cliff and black sand on La Palma, Canary Islands. Chlorite schist is a low-grade metamorphosed mafic igneous rock, often with a basaltic protolith. Iron-bearing green sheet silicate mineral chlorite gives slaty cleavage to the rock. Width of sample 13 cm. Etymology The term “basanite” was already used in antiquity and “basalt” is probably a faulty transcription of basanite. It was German scholar Agricola (Georg Bauer) who first mentioned “basalt” in 1546.

He referred to black columnar rocks from Stolpen (near Dresden in Germany) which is indeed basalt even according to modern classification principles 1 . References 1. Tomkeieff, S. I. (1983). Dictionary of Petrology . John Wiley & Sons. 2. Best, Myron G. (2002). Igneous and Metamorphic Petrology , 2nd Edition. Wiley-Blackwell. Rock Types Rock Types About Rock types Rock type is usually defined as a particular kind of rock having a specific set of characteristics 1 . Rock types are specific assemblages of minerals (most rocks are composed of minerals). Rocks are much more vaguely defined than minerals . Even rocks within one rock type may have a highly variable composition. Some important igneous rocks from left to right: gabbro, andesite, pegmatite, basalt, pumice, porphyry, obsidian, granite, and tuff. Still, there is a fundamentally sound reason to talk about such a vague concept as a rock type because these assemblages occur again and again in many different locations. Rock type is like a biome (desert, savannah, rain forest).

One particular desert may greatly differ from another (one is sandy and another rocky), but they both share something that is common (low precipitation). Exactly the same is true with rock types — one granite may be white and another red, but they both have a similar composition (major minerals are feldspar and quartz). Here is a list of major rock types recognized by geologists. Immense number of rock types have been defined (their definitions often overlapping) and many rock classifications exist. However, there seems to be a fair number of terms and classification principles that have successfully stood the test of time. Contents Igneous rocks Amphibolite · Anthracite · Blueschist · Charnockite · Eclogite · Epidosite · Fenite · Gneiss · Granulite · Greenschist · Greisen · Hornfels · Marble · Metapelite · Migmatite · Mylonite · Phyllite · Quartzite · Schist · Serpentinite · Shungite · Skarn · Slate · Soapstone · Suevite Igneous rocks It all starts with igneous rocks . The Earth is believed to have been entirely molten in its early stages of development.

Hence, all other rock types are derivatives of igneous rocks. Andesite is a volcanic rock with a so-called average composition. It is more felsic than basalt and more mafic than dacite. The main minerals are plagioclase and hornblende. Andesite is a common rock of subduction zone volcanoes. It is generally dark gray and may contain phenocrysts of plagioclase (usually andesine), biotite, and hornblende. Andesite is an extrusive equivalent of diorite. ^ Anorthosite is a plutonic rock which is composed of plagioclase (>90%). Anorthosite is a cumulate rock. Its constituent plagioclase crystals floated to the top of the mafic magma chamber and crystallized there as almost monomineralic plagioclase rock. Some anorthosites display very beautiful display of colors known as labradorescence and are therefore valuable ornamental stones. Anorthosite is a common rock on the Moon. The highlands of the Moon which we see as brighter areas are largely composed of this rock type. ^ Aplite is a fine-grained and usually light-colored plutonic rock.

Aplite usually solidified in a narrow crack where heat loss to the adjacent rocks was rapid and crystals did not have much time to grow in size. Aplites may have a variable composition from granite to gabbro, but the term “aplite” is mostly used to describe fine-grained granitic rocks. ^ Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro. Oceanic crust is mostly composed of basalt or rocks with the same composition (gabbro, diabase). ^ Basanite is a volcanic rock. It is similar to basalt but contains less silica and usually more alkali metals. Basanite may contain olivine, pyroxene, and feldspathoid phenocrysts. Similar rock without olivine is tephrite. Limburgite is a glassy nepheline basanite.

^ Carbonatite is an igneous rock that is predominantly composed of carbonate minerals. Both volcanic and plutonic carbonatites are known. Alvikite is a volcanic calcite carbonatite, sövite is a plutonic calcite carbonatite. Carbonatite is generally associated with alkaline igneous rocks. Carbonatite is a host rock of several important commodities, most notably apatite, rare earth elements and niobium. ^ Dacite is a felsic volcanic rock intermediate in composition between andesite and rhyolite. Dacite is a common rock of subduction-related volcanoes on the active continental margins (Cascades in the northwestern USA, for example). Dacitic magma in the crust solidifies as intrusive rock granodiorite. ^ Diabase is a dark-colored igneous rock. It is compositionally equivalent to gabbro and basalt but texturally between them. Diabase is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. Diabase grades to basalt when it solidifies rapidly and to gabbro when more time is given to the crystals to grow.

^ Diorite is a plutonic rock with sodium-rich plagioclase and hornblende. Diorite is compositionally equivalent to volcanic rock andesite. It is intermediate in composition between felsic granite and mafic gabbro. ^ Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine (>90%). Dunite is a sub-type of peridotite. These rocks are abundant in the Earth’s mantle but occur relatively rarely in continental rocks. Olivine rock is a synonym of dunite. Olivinite is a variety that contains more magnetite than chromite. ^ Foyaite is a rare igneous rock consisting of nepheline and alkali feldspar with a trachytic texture. Foyaite is a variety of nepheline syenite. ^ Gabbro is a coarse-grained and usually dark-colored igneous rock. Gabbro is an intrusive rock. Igneous rocks with a similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead).

^ Granite is a very common plutonic rock with feldspar and quartz as the main constituents. Granite is an igneous rock, but some high-grade metamorphic rocks are compositionally very close and have been sometimes referred to as granites also. ^ Granodiorite is a granitoid (quartz-rich plutonic rock) in which the dominant feldspar is plagioclase. Granodiorite is compositionally between other granitoids granite and tonalite. Granodiorite contains more quartz than diorite. This rock type is frequently misidentified as granite. ^ Harzburgite is what remains when basalt is taken out of the mantle. Harzburgite is a sub-type of peridotite. Harzburgite is composed of orthopyroxene and olivine, it is one of the ultramafic rocks and belongs into the peridotite group. These are rocks that are abundant in the mantle but scarce on the surface. ^ Hawaiite is a volcanic rock similar to basalt but contains more alkali metals. It is a sodic variety of trachybasalt. ^ Hornblendite is an ultramafic plutonic igneous rock that is composed almost entirely of hornblende.

Hornblendite crystallized from water fluid-rich magma which favored the formation of hydrous hornblende over anhydrous pyroxene. ^ Ignimbrite is a fiery rock. It is a pyroclastic rock formed by very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit 3 . ^ Ijolite is a rare igneous rock consisting of nepheline and pyroxene. Ijolite is a variety of foidolite (feldspathoid-rich plutonic rock in which fledspathoids are dominating over feldspars). ^ Kimberlite is an ultramafic rock consisting of olivine and one or more of the following: phlogopite, serpentine, diopside, monticellite, calcite. Kimberlite contains no leucite. Kimberlite is an economically important rock type because it is the principal source (host rock) of diamonds.

Kimberlite occurs in pipe-like rock body called diatreme, which formed through violent gas-charged volcanic eruption. ^ Komatiite is an ultramafic lava. Komatiite is compositionally similar to intrusive rock peridotite. Some volcanic rocks (picrites) may be ultramafic because they are olivine cumulates, but komatiite is a true ultramafic lava that formed as a result of very extensive partial melting. Komatiites do not form anymore. Most likely because the Earth has cooled down and does not allow such a high degree of partial melting to take place. Komatiite lavas commonly demonstrate spinifex texture — long olivine crystals in the upper part of a lava flow. ^ Limburgite is a variety of basanite with a glassy groundmass and nepheline as the main feldspathoid mineral. Limburgite contains no feldspars. ^ Monzonite is a plutonic rock intermediate in composition between syenite and diorite. Monzonite contains approximately equal amount of K-feldspar and plagioclase.

Monzonite contains no or small amount of quartz and the content of alkali feldspar and plagioclase are roughly equal. ^ Nepheline syenite is a plutonic rock rich in alkali metals and relatively poor in silica. As a result of that it contains alkali minerals like feldspathoid nepheline in addition to alkali feldspar, alkali amphibole and/or alkali pyroxene. Quartz is absent. Phonolite is a volcanic analogue of nepheline syenite. ^ Norite is a gabbroid rock in which the pyroxene mineral is orthopyroxene. ^ Obsidian is a volcanic glass — a rock type that in pure form contains no crystals. Obsidian is usually rhyolitic in composition and black or reddish in color. Obsidian is frequently associated with pumice. ^ Pegmatite is an exceptionally coarse-grained plutonic rock. Most pegmatites have a granitic composition. Simple pegmatites contain common minerals like feldspar, mica and quartz. Complex pegmatites are rich in unusual chemical elements and minerals which makes pegmatite a potential mineral resource. ^ Peridotite is the principal rock type in the Mantle.

Unaltered peridotite is usually dark green in color but most peridotites we see in the surface are altered to a metamorphic rock type serpentinite. There are several sub-types of peridotite: harzburgite, dunite, lherzolite, etc. ^ Phonolite is an alkaline volcanic rock that is composed of fine-grained alkali feldspar, nepheline, alkali amphiboles and alkali pyroxenes. Phonolite may be porphyritic. It is an extrusive equivalent of plutonic rock nepheline syenite. ^ Picrite is a mafic igneous rock similar to basalt but with more olivine. Picrite may form as an olivine cumulate rock. Olivine crystals may settle to the lower part of a lava flow, forming an ultramafic rock. ^ Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Porphyritic texture indicates that the crystallization process started well before the lava was extruded to the surface.

^ Pumice is a lava froth — an extremely vesicular and light-weight volcanic rock with a variable composition. Pumice is unusually light-weight for a rock — it floats in water. Pumice is usually light-colored and has a felsic composition. Pumice forms during explosive volcanic eruptions. ^ Pyroxenite is a plutonic rock that is composed almost exclusively (over 90%) of pyroxene minerals. Pyroxenite is a cumulate rock usually associated with other ultramafic (peridotite) and mafic (gabbro) rocks. ^ Rapakivi is a variety of granite with K-feldspar phenocrysts that are mantled with plagioclase. Rapakivi is an anorogenic rock occurring in intra-plate settings. ^ Rhomb-porphyry is a rare porphyritic rock with abundant wedge-shaped anorthoclase phenocrysts. Rhomb-porphyry from Norway is a volcanic equivalent of larvikite. Rhomb-porphyry is compositionally trachyandesite (latite). ^ Rhyolite is a felsic volcanic rock. It is an extrusive analogue of granite.

Rhyolite is associated with explosive volcanoes. Rhyolitic rock, because of high viscosity, may occur as pumice and obsidian. Porphyritic rocks are also frequently rhyolitic in composition. ^ Scoria is a highly vesicular mafic dark-colored volcanic rock. It is similar to pumice but darker in color, not as light-weight, and more mafic in composition. Vesicules in scoria are usually larger than in pumice. ^ Sövite is a type of plutonic carbonatite where the dominant carbonate mineral is calcite. Sövite is the most common variety of carbonatite. It is often mined as a host rock of various mineral resources. ^ Syenite is a plutonic rock that contains little or no quartz and in which K-feldspar dominates over plagioclase. Syenite is like a granite but without quartz. In nepheline syenite some of the alkali feldspar is replaced by nepheline and quartz is completely absent. ^ Tephrite is a volcanic rock resembling basalt, but it is rich in feldspathoid minerals.

This makes it very similar to basanite, but tephrite contains very little olivine. ^ Tonalite is a granitoid in which almost all the feldspar (>90%) is plagioclase. Trondhjemite and plagiogranite are varieties of tonalite. ^ Trachyandesite is a volcanic rock intermediate in composition between trachyte and andesite. Trachyandesite is compositionally close to intrusive rock monzonite. Rhomb-porphyry is a variety of trachyandesite. ^ Trachyte is a feldspar-rich rock. It is a volcanic equivalent of syenite. Trachyte grades into rhyolite with increasing quartz content. ^ Troctolite is a variety of gabbro without pyroxene. Troctolite is a cumulate rock associated with other gabbroic rocks like gabbro, norite, and anorthosite. ^ Trondhjemite is a leucocratic variety of tonalite composed of sodic plagioclase, quartz and biotite. ^ Tuff is a volcanic analogue of sandstone. It is a consolidated and lithified volcanic ash.

^ Sedimentary rocks Sedimentary rocks are products of the consolidation of loose sediments which are in most part bits and pieces of other disintegrated rocks. There are three main types of sedimentary rocks: mudstone, sandstone, limestone. Sure, this is very simplistic approach, but volumetrically almost all sedimentary rocks fall under these categories if we give them relatively broad definition e.g. sandstone also includes siltstones and conglomerates, and limestone includes all sedimentary carbonate rocks. There are notable exception that do not fit into this classification scheme like evaporites, coal, and chert, but volumetrically they are clearly less important. In the list below, of course, these exceptions are not neglected. Arkose is a feldspar-rich (25%) sandstone. Arkose is usually coarse-grained and composed of angular grains. These properties are characteristic of relatively immature sediments. Arkose is usually a disintegration product of nearby granitic rocks. ^ Banded iron formation is a chemogenic metasedimentary rock consisting of alternating silica- and iron-rich bands. Banded iron formation (BIF) is the principal source of iron. It is perhaps the most important rock type as our society is heavily reliant on iron extracted from it.

^ Bauxite is an aluminum-rich sedimentary rock. It is a principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks. ^ Bog iron is a type of iron ore composed of limonite with impurities like clay or plant debris. Bog iron is porous rock formed in bogs, wetlands, and shallow lakes. Iron is precipitated from water by the oxidizing action of bacteria and algae. Bog iron is common in cold temperate zones of Scandinavia and North America. ^ Breccia is a coarse-grained shattered rock that is composed of highly angular fragments held together by a mineral cement. Breccia may be a result of several different geological processes: talus formation, explosive volcanic eruptions, tectonic movements, rock avalanches, etc. ^ Chalk is a soft, friable and porous variety of limestone. Chalk is composed of calcareous tests of marine microorganisms like coccoliths and foraminifers.

The best known chalks are of Cretaceous age. Chalk is often associated with chert (flint). ^ Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. Chert occurs mostly in carbonate sedimentary rocks as nodules or layers. Flint is a dark-colored variety of chert. ^ Coal is a combustible rock containing more than 50% by weight of carbonaceous material. Coal in most cases is a lithified peat. Coal is a major fossil fuel, although in recent years its importance in the developed world has been declining. ^ Conglomerate is a lithified gravel (rounded rock fragments or mineral grains larger than 2 mm in size). Conglomerate contains smaller grains (matrix) in the interstices. Conglomerate is a former river bed, cobble beach or another environment where the constituent clasts were rounded. Breccia is somewhat similar rock with angular clasts. ^ Coquina is a detrital limestone consisting of shells or shell fragments. Coquina is a coarse-grained (particles over 2 mm in size) conglomeratic rock. Finer carbonate rock made of fossil fragments is calcarenite.

^ Diamictite is a poorly sorted or non-sorted terrigenous non-calcareous sedimentary rock. Diamictite is composed of widely variable clast sizes from clay to boulders. Diamictite is a nongenetic term (purely descriptive, no formation mechanism is implied). Most diamictites are glacial in origin (tillite), but some are volcanic (lithified lahars), submarine (turbidites), mass wasting breccias, etc. ^ Diatomite (diatomaceous earth) is a light-colored sedimentary rock composed chiefly of siliceous shells (frustules) of diatoms. Diatomite is soft and friable rock. Most diatomite deposits are marine but some were formed in lakes. Diatomite has several industrial uses. ^ Dolomite (dolostone) is a carbonate rock containing more than 50% by weight of the mineral dolomite. Dolomite may be a reservoir rock for hydrocarbons (oil & gas). ^ Evaporite is any rock type formed as a result of consolidation of salts crystallized from hypersaline water.

Evaporites may form in lagoons where evaporation is intensive due to hot climate and the influx of new lower salinity water is (periodically) restricted, but in many cases evaporites formed by replacing the original minerals of sedimentary rocks with the new ones from brines percolating the rocks. Rock salt and gypsum (as a rock type) are examples of evaporites. ^ Flint is basically a synonym of chert. The term is especially often used for a homogenous dark gray or black variety of chert. Flint is more widely used name for the rock in general public, but the term “chert” is usually preferred in geological literature (regardless of the color of the rock). ^ Graywacke is an old and vaguely defined rock name for a coarse-grained firmly indurated sandstone that is dark-colored and contains lots of clay and lithic fragments. Graywacke is poorly sorted marine rock. It is nowadays interpreted to be of turbiditic origin.

This rock name has been extensively criticized since 1839 when Roderick Murchison said that “it has already been amply shown that this word should cease to be used in geological nomenclature, and…is mineralogically worthless”. Almost two centuries later many geologists seem to share the view of Murchison. But regardless of that, the rock name continues to be used at least informally and as a field term for a dark-grained “dirty” sandstone about which “you can’t tell much about in the field”. ^ Gypsum as a rock type is an aggregate of gypsum crystals. Gypsum is an evaporite and is usually associated with other evaporite minerals like halite and anhydrite. Gypsum occurs in different forms: massive alabaster, fibrous satin spar, selenite crystals. Gypsum is an important industrial mineral. It is used mostly in the construction industry. ^ Laterite is a reddish weathering product of basaltic (mostly) rocks. Laterite forms in hot to temperate climate and is a residual product of weathering. ^ Lignite is a variety of low-quality coal that is intermediate in coalification between coal and sub-bituminous coal.

Brown coal is a synonym of lignite. ^ Limestone is a sedimentary rock that is chiefly composed of calcium carbonate. Limestone is a major rock type. It is mostly marine and biogenic, but can also form as a chemical precipitate (travertine, tufa). Oolite, chalk, calcarenite, coquina, etc. are varieties of limestone. ^ Oil shale is a rock that yields oil (shale oil) upon heating without oxygen. Organic (burning) matter in oli shale is kerogen. Oil shale is a variety or shale. Oil shale reserves are abundant, but it is seldom used as a fossil fuel because it is usually low in calorific value and contains lots of impurities. ^ Oolite is a rock made up of small, generally rounded grains with internal growth bands that are named ooids. Most oolites are limestones (ooids are composed of calcium carbonate). ^ Phosphorite is a sedimentary rock containing significant amount of phosphate material. Phosphorite is a rock containing enough phosphate to be of economic interest. Phosphatic matter may be in ooids, pellets, nodules, shells, bones, etc.

^ Sandstone is a lithified sand. There are many subtypes of sandstone: arkose, orthoquartzite, grit, flagstone, graywacke, and many others. Sandstone is among the most widespread sedimentary rocks. The most abundant constituent mineral in most sandstones is quartz. ^ Shale is a laminated rock consisting of mostly clay minerals. Shale is a lithified mud. Shale and similar mudstones together make up more than half of all the sedimentary rocks. Shale is a source rock of crude oil and natural gas. ^ Siltstone is an intermediate member between sandstone and shale in both grain size and properties. Loess is a variety of wind-deposited siltstone. Pure siltstone is composed of very fine dusty grains, but it may contain significant amount of sand and clay. ^ Tillite is a poorly sorted glacial sedimentary rock — consolidated glacial till. Tillite in many cases is referred to as diamictite, although the latter is purely descriptive term and may have several formation mechanisms. ^ Travertine is a chemically precipitated limestone. Stalactites and other speleothems in karst caves are made of this rock type.

Travertine is also filling calcitic veins. Tufa is a soft and porous chemically precipitated calcitic rock. ^ Turbidite is not a rock type in a strict sense. It is a sedimentary complex consisting of alternating layers of muddy and silty sediments. Turbidite is inferred to have been deposited underwater by a turbidity current. Flysch is a frequently encountered term in older geological literature that means the same thing. ^ Umber is a marine sediment associated with black smokers. Umber is brown in color (brown earth, darker than ocher) and valued as a natural brown pigment. Umber occurs in Cyprus-type massive sulfide deposits. ^ Metamorphic rocks Metamorphic rocks are derived from other pre-existing rocks by mineralogical and/or structural changes. Metamorphism takes place in the solid state and at elevated pressure and temperature generally at depth in the crust 1 . Amphibolite is a metamorphic rock composed of amphiboles (usually hornblende) and plagioclase. Amphibolite should not be confused with the metamorphic facies of the same name. Amphibolite is a widespread rock type.

^ Anthracite is a type of coal with a highest rank. Coal rank measures the carbon content which is in correlation with calorific value and metamorphic grade. Anthracite has a very high carbon (over 90%) and low volatile content (below 5%). ^ Blueschist is a bluish metamorphic rock containing amphibole glaucophane. Blueschist forms at high pressure and low temperature (subducting slab of lithosphere). Its protoliths are basaltic rocks. ^ Charnockite is an orthopyroxene-bearing anhydrous metamorphic rock. Charnockite is often referred to as granite. This practice is in most cases misleading because charnockitic rocks did not crystallize from magma. ^ Eclogite is a beautiful rock consisting of bright green pyroxene omphacite and red garnet. Eclogite forms at high-pressure conditions. Its protolith is a basaltic rock. ^ Epidosite is a hydrothermally altered basaltic rock consisting of epidote and quartz. Epidosite is a product of hydrothermal metamorphism (alteration). Its parent rock was a mafic igneous rock (basalt, diabase).

^ Fenite is a metasomatic rock surrounding complexes of carbonatite-alkaline rocks. Fenite is an altered former granitic (or similar) rock. Fenitization is a process which replaces some or all of the original minerals by new and generally more alkaline phases (alkali pyroxene, alkali amphibole, feldspathoids, etc.). ^ Gneiss is a banded coarse-grained metamorphic rock that formed deep in the crust beneath forming mountain ranges. The chemical composition may be highly variable, although most gneissose rocks are granitic. ^ Granulite is a confusing term in metamorphic petrology. Usually it is a metamorphic rock of granulite facies. This is unfortunate use because metamorphic rocks should never be confused with facies names. There is a similar case with amphibolite and amphibolite facies, but amphibolite is a well-defined rock type. We know that amphibolite does not necessarily equilibrate at amphibolite facies conditions and neither are all amphibolite facies rocks amphibolites. The situation, unfortunately, is more complicated with granulites because there are no compositional defining driteria.

Therefore, rocks with widely varying compositions are named so which complicates the matter to the point of utter confusion. It has been proposed that the term should be abandoned and granofels used instead which is free of unwanted associations with metamorphic facies. Nevertheless, the term “granulite” continues to be used in the scientific literature. ^ Greenschist is a schistose metamorphic rock. Green color is given to this rock usually by chlorite, epidote, actinolite, etc. Greenschist is usually metamorphosed mafic igneous rock (metabasite). If the green mineral is chlorite, the rock should be named chlorite schist. ^ Greisen is a pneumatolytically altered granitic rock. Greisen contains lots of quartz and mica plus several exotic minerals like cassiterite, tourmaline, topaz, etc. Greised is a metamorphic rock that metamorphosed itself — hot fluids that altered the rock were provided by the cooling magma body itself. ^ Hornfels is a fine-grained compact rock rock formed by contact metamorphism (low pressure, high temperature).

^ Marble is a metamorphosed (recrystallized) carbonate rock (limestone, dolomite rock). Marble is mostly composed of calcium carbonate. Marble in geology has somewhat more restrictive meaning than in commerce. ^ Metapelite is a metamorphosed claystone (mudstone, shale). Usually either schist or slate is named so although any metamorphosed rock can be metapelite if it has a clayey protolith. These rocks are rich in aluminum and contain various Al-rich silicate phases like micas, garnet, staurolite, kyanite, cordierite, etc. ^ Migmatite is an inhomogenous rock consisting of dark-colored metamorphic rock and light-colored igneous leucosomes. Igneous or igneous-looking component of the rock is either the result or partial melting or material intruded into the rock. ^ Mylonite is a highly deformed rock that is composed of stretched and flattened mineral grains. Mylonites form deep in the crust where temperature is high enough for ductile deformations in shear zones. ^ Phyllite is a metamorphic rock (usually metapelite) intermediate in grade between slate and schist.

Minute platy crystals of sericite, chlorite, or graphite give a characteristic sheen to the cleavage surfaces which distinguishes it from duller slate. Constituent minerals in schist are coarser. ^ Quartzite is a metamorphic rock consisting largely or entirely of quartz 2 . In vast majority of cases, quartzite is a metamorphosed sandstone. ^ Schist is a strongly foliated rock rich in platy or elongated minerals. Schist forms during regional metamorphism. Composition may be highly variable. ^ Serpentinite is a metamorphosed ultramafic rock. It is composed of serpentine minerals. Serpentinite forms at the expense of olivine- and pyroxene-bearing rocks (peridotite, pyroxenite, dunite). Chlorite, talc and magnesite frequently occur in these rocks. ^ Shungite is a metamorphosed oil shale consisting of structureless carbon. Shungite is a very rare rock type and the details of its genesis are unclear. ^ Skarn is a metasomatic rock composed of calc-silicate minerals. Skarn is formed at the contact zone between carbonate rocks and silicic magma.

Skarn is often rich in hydrothermal ore minerals. ^ Slate is a fine-grained rock that can be split into slabs (slaty cleavage). Slate in the majority of cases is a compacted and metamorphosed mudstone or shale. Slate is a low- to medium-grade metamorphic rock that will pass on to phyllite, schist and gneiss with increasing burial depth. ^ Soapstone is a soft rock composed of talc with other silicates like micas, pyroxene, amphibole, chlorite, etc. Soapstone is named so because of slippery feel when touched. Soapstone may be schistose (talc schist) or massive. ^ Suevite is a shock-metamorphosed rock. Suevite is an impact breccia composed of impact melt and lithic fragments. Impact metamorphism is caused by a collision between Earth and large extraterrestrial body which is usually meteorite or asteroid. ^ References 1. Jackson, J. A. (1997). Glossary of Geology , 4th Edition. American Geological Institute. 2. Barth, T. F. W. (2007). Quartzite.

In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 14. 722. 3. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 9. 20-21. basalt | rock | Britannica.com Basalt Basalt, extrusive igneous (volcanic) rock that is low in silica content, dark in colour, and comparatively rich in iron and magnesium . Basalt. Synek125 Some basalts are quite glassy (tachylytes), and many are very fine-grained and compact. It is more usual, however, for them to exhibit porphyritic structure, with larger crystals (phenocrysts) of olivine , augite , or feldspar in a finely crystalline matrix (groundmass). Olivine and augite are the most common porphyritic minerals in basalts; porphyritic plagioclase feldspars are also found.

Basaltic lavas are frequently spongy or pumiceous; the steam cavities become filled with secondary minerals such as calcite , chlorite, and zeolites. Basalts may be broadly classified on a chemical and petrographic basis into two main groups: the tholeiitic and the alkali basalts. Tholeiitic basaltic lavas are characterized by calcic plagioclase with augite, pigeonite or hypersthene, and olivine (rarely) as the dominant mafic minerals; basalts without olivine are also well represented. Tholeiitic basalts, which contain 45 to 63 percent silica, are rich in iron and include the tholeiites (basalts with calcium-poor pyroxene). They predominate among the lavas of mountain belts; their flows may build enormous plateaus, as in the northwestern United States , the Deccan of India , and the Paraná Basin of South America. The active volcanoes of Mauna Loa and Kilauea in Hawaii erupt tholeiitic lavas.

Normal alkali basalt contains olivine and, commonly, a diopsidic or titaniferous augite. Alkali basalts predominate among the lavas of the ocean basins and are common among the mafic lavas of the forelands and backlands of the mountain belts. In the Brito-Icelandic province the Paleogene and Neogene lava flows of the Inner Hebrides , Antrim, and the Faroe Islands include great successions of both tholeiitic and alkali basalts. Minerals of the feldspathoid group occur in a large number of basaltic rocks belonging to the alkali group; nepheline , analcime, and leucite are the commonest, but haüynite is occasionally present. If nepheline entirely replaces feldspar, the rock is known as nepheline-basalt; if the replacement is only partial, the term nepheline-basanite is used. Similarly, there are analcime- and leucite-basalts and leucite-basanites.

Most nepheline-basalts are fine-grained, very dark-coloured rocks and date to the early Cenozoic Era (65.5 million years ago to the present). They are fairly common in some parts of Germany and also occur in the United States (as in New Mexico) and in Libya, Turkey , and elsewhere. Leucite-basalts are found principally in Italy , Germany, eastern Africa , Australia, and, in the United States, in Montana, Wyoming, and Arizona. Learn More in these related articles: in dating (geochronology) Basalt Rocks Basalt Rocks Introduction Basalt rocks are the most common dark-colored igneous rocks you will find. They contain ferrogmanesian minerals which are iron and magnesium rich. The shield volcanoes of Hawaii, cinder cones in the Pacific Northwest and the floors of the world's oceans are made of basalt. Pillow basalt on the ocean floor, NOAA Basalt Basalt is a dark heavy lava that is very hot and fluid when it erupts. It it erupts on the continents it can flow many kilometers before it cools in thin layers.

Flood basalts Flood basalts made up of thin layers of lava flows cover thousands of square miles in Oregon and Washington. The layers of lava are over three miles thick near Yakima, Washington. Layers of flood basalts are exposed in the scenic Columbia George that separates Washington and Oregon. Cinder cones Cinder cones are small cone shaped volcanoes created entirely of basalt rocks blown out of a volcano. The magma beneath the volcano vent contains large amount of molten rock and gas mixed together under pressure. Cinders (scoria) The cinders (scoria) are erupted during small explosive eruptions which creates steep-sided volcanoes. Most of the cinders on the volcano are about the size of a golf ball or walnut. Cinder cones are also called scoria cones by geologists in some areas. Lava Bombs Lava bombs are larger pieces of lava that were partly molten during the eruption. These rocks take on aerodynamic shapes while they are airborne. In general all the rocks that form a cinder cone are called cinders no matter how large or small the rocks are that formed during an eruption.

More Rock Cycle Links Salt Domes Learn more about how salt domes form, what they are used for and how they are sometimes a key to find oil. Lava Caves Find out how pahoehoe lava flows form lava caves. Ignimbrite Learn more about ignimbrite that forms during large volcanic eruptions that produce pyroclastic flows. Coal Formation Find out how leaves falling into a swamp can turn into coal. Pahoehoe-Lava Find out how pahoehoe lava forms in the Hawaiian Islands. Basalt Rocks These rocks cover the floors of all the oceans, create cinder cones and form the largest volcanic mountains on Earth. Rock Cycle Learn more about igneous, sedimentary and metamorphic rocks and how they form. Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.

Question: What type of rock is basalt?
Answer: Igneous